Wavelength converters, such as phosphors, are commonly used to provide light comprising a plurality of wavelengths from a single wavelength light emitting element. The composite light output is commonly a combination of the light emitted from the converters upon absorption of some of the light emitting from the light emitting element, and the remainder of the non-absorbed light from the light emitting element.
In an example embodiment such as illustrated in FIG. 1A, the light emitting device 101 includes a light emitting element 110 that emits blue light and a ceramic element 120 above it that includes blue-to-green wavelength converter material. For ease of reference, a wavelength converter that emits green light, irrespective of the wavelength of the light absorbed by the converter, is hereinafter termed a green converter, the individual elements, or particles, that effect the conversion being termed green converter material. When the green converter material in the ceramic element 120 absorbs the blue light from the light emitting element 110, they emit green light. The composite light is closer to ‘white’ light than the blue light that is emitted from the light emitting element, and may be used for select applications, such as automotive headlights, that currently allow for a cool white illumination.
The ceramic element 120 allows for a fairly uniform and consistent distribution of green converter material through the element 120. The rigid nature of the ceramic composition of element 120 serves to provide permanence to this distribution of green converter material in element 120. The surface 125 of the ceramic element 120 may also be roughened to enhance the light output efficiency without significantly affecting the structure and characteristics of the element 120. The rigidity of the ceramic element 120 also facilitates the handling of the element for processes such as a pick-and-place process that situates the ceramic element 120 atop the light emitting element 110.
The light emitting element 110 with ceramic element 120 is encased in a reflective material 140, such as silicone loaded titanium oxide, TiO. The reflective material 140 serves to protect the light emitting device 101 and to reflect side emitted light from light emitting element 110 and ceramic element 120 to the surface 125 of the ceramic element 120, to provide a higher projection contrast (well defined boundary between illuminated and non-illuminated regions), which is preferred in certain applications, such as automotive lighting.
In a typical operating environment, the light emitting device 102 is coupled to a heat sink 160, which absorbs and dissipates the heat generated by the light emitter 110 and the green converters in the ceramic element 120. The ceramic element 120 is an efficient thermal conductor, but the surrounding atmosphere is not, and therefore only a small amount of the heat that is generated in the ceramic element 120 is dissipated. The heat that is generated by the conversion of blue light to green light will be conducted through the glue layer 115 and blue emitter 110 to the heat sink 160.
Although the composite light output of the light emitting device 101 is closer to a white output than the blue light from the light emitting element 110, some applications require a light output having a ‘warmer’ color temperature.
To provide a light that appears ‘more white’ (having a warmer color temperature) than the cool white light output of the device 101, a wavelength conversion element that emits red light (‘red converter’) may be added, as illustrated in the device 102 of FIG. 1B. Current technology, however, does not allow for red conversion material to be embedded in a ceramic element, and therefore a separate coating, such as a red converter material loaded silicone layer 130, is applied between the (blue) light emitting element 110 and the ceramic (green) converter element 120.
Although this arrangement of blue-red-green emitting elements of device 102 has a number of advantages, including providing a durable external surface 125, and a relatively high red converter efficiency, being adjacent the blue light emitting element 110, it exhibits poor thermal dissipation efficiency.
As noted above, although the ceramic layer 120 may dissipate some heat through the upper surface 125 to the surrounding atmosphere, the thermal transfer efficiency at this interface is poor. Compounding the matter, red converters generate more heat than green converters, having to effect a larger wavelength conversion. Although some of this heat will be transferred to the ceramic element 120, the ceramic element cannot efficiently dissipate this additional heat.
Accordingly, most of the conversion-generated heat must travel through the red converter layer 130 and the glue layer 115 to the emitter 110 and heat sink 160. The red converter layer 130 and the glue layer 115 are commonly silicone based, and silicone has a relatively low thermal transfer efficiency. Thus, the layers 130 and 115 will act as a thermal barrier to this required heat transfer. Thus, the expected operating temperature of the device of FIG. 1B will be high, increasing the likelihood of failure and reducing the device's life span.